The Stanbridge Nappe in Vermont: a Reappraisal of Its Allochtho
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A2 - 1 EVIDENCE AGAINST THE ALLOCHTHONOUS NATURE OF THE STANBRIDGE NAPPE AT HIGHGATE GORGE, NORTHWESTERN VERMONT Adam Schoonmaker, Department of Geosciences, Utica College, Utica, NY 13502 William S.F. Kidd, Atmospheric and Environmental Sciences, University at Albany, Albany, NY 12222 INTRODUCTION This trip features the spectacular exposure of upper Cambrian and Lower Ordovician carbonate shelf and shelf edge strata and overlying limestone breccias and calcareous shales in the Highgate Falls Gorge, at Highgate Center, northwestern Vermont. Here, a continuously exposed, conformable sequence of sandy dolomitic breccias of the Gorge Formation and shaly limestones and limestone breccias of the Highgate Formation are overlain by partly black slates and minor micrites of the Morses Line Formation. This contrasts with the conclusions reached by workers in southern Quebec, where this sequence is inferred to contain a major tectonic (thrust) boundary between overlying allochthonous slaty limestone, calcareous slate, and argillite (Highgate and Morses Line Formations) and underlying dolomite and dolomite breccia of the Laurentian shelf sequence (the Milton Dolomite in Quebec, which is in part equivalent to the Gorge Formation of Vermont). We shall examine the relationship of these rocks in the gorge and at selected stops to the north (Rock River and Choiniére Farm), near the International Border. Also, beautifully exposed in the gorge, is the Highgate Falls Thrust, an out-of-sequence thrust fault that is likely part of the larger Champlain Thrust system, and which emplaces upper Cambrian dolomite breccias of the Gorge Formation over lower to mid-Ordovician black slates of the Morses Lines Formation. Diachronously evolved en echelon fractures sets (many showing various stages of rotation) and associated minor thrusts are well-displayed throughout the slates beneath the Highgate Falls Thrust. Walking in the gorge can be treacherous and difficult both on the boulder field in the channel and the outcrop; both may be very slippery if wet. Care should be observed, and adequate footwear is advised. We include additional stops at the Swanton Limestone Quarry, the “Beam”, and Lessor’s Quarry. The Swanton quarry exposes of the top surface of the lower block (Iberville Formation) of the Highgate Springs Thrust, part of the Champlain Thrust fault system. The latter two stops have been designated as “Rolfe Stanley Memorial Outcrops” by the University of Vermont and beautifully display foreland deformation features in parautochthonous Laurentian shelf rocks (NO HAMMERS FOR THESE TWO STOPS, PLEASE) THE STANBRIDGE NAPPE OF SOUTHERN QUEBEC Adjacent to the International Border, in southern Quebec (Figure 1), the east-dipping Ordovician-aged Stanbridge Nappe has previously been interpreted to be an allochthonous group of carbonates and argillaceous slates derived from the Laurentian continental rise, which was detached and thrust over the upper Cambrian Milton Dolomite, part of the imbricated, parauthochthonous Laurentian shelf, along a major, structural boundary (St. Julien and Hubert, 1975; Charbonneau, 1980; Globensky, 1981). Although it is not exposed, the western boundary of the Stanbridge Complex comprises the southernmost section of Logan’s Line in Quebec. The Stanbridge Nappe is the southernmost of the Quebec Allochthons, part of the larger belt of allochthons that extend discontinuously from Newfoundland, across to the Gaspé and southwards to just across the International Border in northwestern Vermont; The allochthons reappear in west-central Vermont and continue southwards as the Taconic Allochthons. They are generally composed of far-traveled low-grade metamorphosed deep-marine mud- rocks and clastics originally deposited on the Laurentian continental lower slope and rise, and thrust westward, up and over the Laurentian shelf. The two belts are separated by parauthochthonous carbonate and siliciclastic rocks deposited on the Laurentian shelf and upper slope, that was subsequently imbricated during the Taconic Orogeny (e.g. the Champlain Thrust). While these parauthochthonous rocks have undergone transport along thrust faults, they are still in structural contact with related rocks deposited in a similar setting (e.g. the continental shelf), This contrasts with the allochthons that have seen significant transport from an original lower slope and rise setting and are now structurally emplaced against shelf rocks. Many of the Quebec allochthons are floored by flysch and contain olistostromal units, and all are structurally emplaced on top of younger rocks. SCHOONMAKER AND KIDD A2 – 2 Figure 1. Regional map of significant structures and lithologic units in northwestern Vermont and southern Quebec. Based in part on Doll et al. (1961), Fisher (1968), Charbonneau (1980), Globensky (1981), and Avramtchev (1989). SCHOONMAKER AND KIDD A2 – 3 The Stanbridge Nappe is composed of the dominantly argillaceous Stanbridge Complex of Charbonneau (1980; Figure 1; Figure 2). The complex is divided into three sequences: 1) the lower sequence, composed of bedded slaty limestone and limestone conglomerates, overlain by thick sequences of bedded calcareous slate with sparse individual limestone beds and ribbon limestone bed sequences (usually not more than a few meters thick); 2) an intermediate rhythmite unit, composed of thinly laminated siltstone-argillite-mudstone beds; and 3) an upper sequence of calcareous slate, slaty limestones and calcareous conglomerates. The entire complex is an internally coherent package that structurally overlies massive dolomites, chert-bearing and sandy dolomites, and dolomitic conglomerates of the Milton Dolomite along an unnamed (and unobserved) thrust fault. The Milton Dolomite (a term abandoned south of the International Border) is the northern extension of the Dunham Dolomite, and Saxe Brook and Gorge formations of northwestern Vermont, and is part of the imbricated carbonate-siliciclastic shelf sequence. The inferred structural relationship between the slaty Stanbridge Complex and underlying non-slate- bearing shelf carbonates is first reported in St. Julien and Hubert (1975) who include the bedded limestones of the lower sequence of the Stanbridge Complex as part of the transported Stanbridge Nappe. Significantly, the Stanbridge Complex does not contain flysch or olistostromes, and structurally overlies older, or approximately coeval rocks of the Rosenberg and Phillipsburg slices (Figure 1). CORRELATIVE ROCKS IN NORTHWESTERN VERMONT In Vermont, the lower slaty limestones and overlying calcareous slates of the lower unit of the Stanbridge Nappe are correlated with the Highgate and Morses Line Formations, respectively (Figure 4; Charbonneau, 1980; Globensky, 1981; Schoonmaker, 2005). The intermediate rhythmite and upper sequence correlate with higher sections of the Morses Line Formation above the Corliss Conglomerate, an internal member of the Morses Line Formation. Underlying the Highgate Formation are a series of massive dolomites, sandy dolomites, and dolomite breccias, including the Dunham, Saxe Brook, and Gorge Formations (in ascending order), all part of the Rosenberg Slice of Clark (1934) and equivalent to the Milton Dolomite in Quebec (Figure 3). These dolomitic units beneath the Highgate Formation have long been assigned to the imbricated shelf sequence (e.g. Stanley and Ratcliffe, 1985). Previous workers in Vermont are divided on the presence of a major structural boundary in this section. Mehrtens and Dorsey (1987), and Schoonmaker and Kidd (2007) have interpreted the contacts between the Gorge and Highgate and Highgate and Morses Line Formation to be conformable, and it is similarly shown on the Centennial Map of Doll et al. (1961). Shaw (1958) and Pingree (1982) placed a thrust fault at the contact between the Highgate Formation and Morses Line Slates, while Haschke (1994) placed a normal fault at that same position. However, all these workers concluded that that bedded limestones and limestone breccias of the Highgate Formation were deposited on top of the dolomites and dolomitic breccias of the Gorge Formation. This contrasts with the interpretation in Quebec where the base of the bedded limestones and limestone breccias (Highgate Formation) is interpreted to be in thrust fault contact with the underlying dolomites and dolomitic breccias of the Milton Dolomite (Gorge Formation). We will observe the contact relationships between the dolomitic units of the Gorge Formation and overlying bedded limestones and limestone breccias of the Highgate Formation (stop 1a), as well as the overlying partly calcareous slates, previously referred to as the Highgate Slate (e.g. Keith, 1923), but which we reclassify as the lower part of the Morses Line Formation. These are beautifully exposed in the Highgate Falls Gorge (Stop 1b; Figures 3 and 5). We will also observe rocks lithologically similar to those seen in the gorge that are present to the north-northeast, well east of the strike of the westernmost Stanbridge rocks in Quebec (Stops 2 and 3). The relationships we will observe show that the Highgate and Morse Line formations are internally conformable (with the exception of minor thrusts, and the younger, out-of-sequence Highgate Falls Thrust) and depositionally overlie the shelf-derived dolomitic Gorge Formation. Also, we will visit the Swanton Limestone Quarry in Swanton (Stop 4), where large sections of the hanging wall to the Highgate Springs Thrust (part of the Champlain Thrust system) have been removed, spectacularly